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How do these vendors and buyers feel about being photographed? Do some of them grant permission and then try to demand money afterward?
They are pretty relaxed about it. They certainly wouldn't demand money. They seem more amused than anything else.
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How is the sanitation in terms of food safety and parasites? Do you have to take special precautions to clean greens or fruit that you don't peel?
I never buy from these unlicensed traders. I'm not saying their stuff is bad, but it is unchecked. The licensed traders inside the market proper are checked fairly regularly, especially before holidays and festivals, all of which are celebrated by eating till you drop..
Any vegetables and fruit, no matter where you buy them need careful and thorough cleaning. China still relies extensively on 'night soil" as a fertiliser., This is human excrement gathered from public conveniences, local schools and colleges factories etc. Full marks for recycling, but...
There is also an over-reliance of chemical fertilisers in recent years.
Chinese people never eat the skin on fruit. Most people, including me, carry little knifes just to peel an unexpected apple or pear.
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Are those bananas or plantains in that picture?
Bananas. In the USA and UK we tend to only get Cavendish bananas (we also get them here) . In SE Asia there are many varieties, I don't know the names in English.